diff --git a/src/ssn/readSolidpkg.ts b/src/ssn/readSolidpkg.ts
index 717ef91..f475008 100644
--- a/src/ssn/readSolidpkg.ts
+++ b/src/ssn/readSolidpkg.ts
@@ -85,7 +85,8 @@ export default function readSolidpkg(buffer: Buffer) {
 
   //read password from extra field
   let encodedPassword: Uint8Array | undefined;
-  while (pos + 4 <= extraFieldLength) {
+  const extraFieldEnd = pos + extraFieldLength;
+  while (pos + 4 <= extraFieldEnd) {
     const fieldId = dv.getUint16(pos, true); pos += 2;
     const fieldLength = dv.getUint16(pos, true); pos += 2;
     switch (fieldId) {